Chaining Commands

Linux Luminarium

In the piping module, you've explored the concept of using several commands, with data flowing between them via pipes, to accomplish something slightly more complex than the individual commands can do. Of course, this concept also applies independent of the data transfer: sometimes, you might want to run several commands in quick succession to achieve up some cumulative effect.

This module will cover a few ways, aside from piping, that commands can be chained. By the end, you'll be on your way to writing shell scripts!


The easiest way to chain commands is ;. In most contexts, ; separates commands in a similar way to how Enter separates lines. So, this:

hacker@dojo:~$ echo COLLEGE > pwn
hacker@dojo:~$ cat pwn

Is roughly the same as this:

hacker@dojo:~$ echo COLLEGE > pwn; cat pwn

Basically, when you hit Enter, your shell executes your typed command and, after that command terminates, give you the prompt to input another command. The semicolon is analogous, just without the prompt and with you entering both commands before anything is executed.

Give it a try now! In this level, you must run /challenge/pwn and then /challenge/college, chaining them with a semicolon.

As you combine more and more commands to achieve complex effects, the length of the combined prompt quickly gets really annoying to deal with. When this happens, you can put these commands in a file, called a shell script, and run them by executing the file! For example, consider our semicolon technique:

hacker@dojo:~$ echo COLLEGE > pwn; cat pwn

We can create a shell script called (by convention, shell scripts are frequently named with a sh suffix):

echo COLLEGE > pwn
cat pwn

And then we can execute by passing it as an argument to a new instance of our shell (bash)! When a shell is invoked like this, rather than taking commands from the user, it reads commands from the file.

hacker@dojo:~$ ls
hacker@dojo:~$ bash
hacker@dojo:~$ ls

You can see that the shell script executed both commands, creating and printing the pwn file.

Now, it's your turn! Same as last level, run /challenge/pwn and then /challenge/college, but this time in a shell script called, then run it with bash!

NOTE: We haven't yet talked about Linux's amazing array of competent command line file editors. For now, feel free to use the Text Editor application in Desktop mode (Applications->Accessories->Text Editor) or the default editor in the VSCode Workspace!

Let's try something a bit trickier! You've piped output between programs with |, but so far, this has just been between one command's output and a different command's input. But what if you wanted to send the output of several programs to one command? There are a few ways to do this, and we'll explore a simple one here: redirecting output from your script!

As far as the shell is concerned, your script is just another command. That means you can redirect its input and output just like you did for commands in the Piping module! For example, you can write it to a file:

hacker@dojo:~$ cat
echo PWN
hacker@dojo:~$ bash > output
hacker@dojo:~$ cat output

All of the various redirection methods work: > for stdout, 2> for stderr, < for stdin, >> and 2>> for append-mode redirection, >& for redirecting to other file descriptors, and | for piping to another command.

In this level, we will practice piping (|) from your script to another program. Like before, you need to create a script that calls the /challenge/pwn command followed by the /challenge/college command, and pipe the output of the script into a single invocation of the /challenge/solve command!

You have written your first shell script, but calling it via bash is a pain. Why do you need that bash?

When you invoke bash, you are, of course launching the bash command with the argument. This tells bash to read its commands from instead of standard input, and thus your shell script is executed.

It turns out that you can avoid the need to manually invoke bash. If your shell script file is executable (recall File Permissions), you can simply invoke it via its relative or absolute path! For example, if you create in your home directory and make it executable, you can invoke it via /home/hacker/ or ~/ or (if your working directory is /home/hacker) ./

Try that here! Make a shellscript that will invoke /challenge/solve, make it executable, and run it without explicitly invoking bash!


This scoreboard reflects solves for challenges in this module after the module launched in this dojo.

Rank Hacker Badges Score